French patent FR-2,814,378 describes a natural gas pretreating method allowing to obtain, at a low cost, a methane-rich and hydrogen sulfide-depleted gas substantially free of all the water that said natural gas initially contained. A hydrocarbon-depleted aqueous liquid containing a large part of the hydrogen sulfide is obtained in parallel and generally injected into an underground reservoir, an oil production well for example. Thus, the method described in this French patent allows, within a single stage, to remove or to significantly reduce the water initially contained in the natural gas while reducing the sour constituent contents. The method described in this patent also allows to obtain a liquid phase containing mainly hydrogen sulfide, which can be readily pressurized and injected into the well. However, the method of French patent FR-2,814,378 does not allow to reduce the hydrogen sulfide and carbon dioxide content of the gas thus treated to an acceptable level as regards commercial requirements. It is therefore often necessary to reduce this sour gas content by post-treating. The methods generally used for these post-treatments are chemical absorption methods using, for example, solvents containing amines, at high temperatures or temperatures close to the ambient temperature. These post-treating methods allow deacidizing of the natural gas the chemical solvent absorbs the sour constituents by chemical reaction. However, they have the drawback of charging the deacidized gas with water because of the use of the chemical solvent in aqueous solution. Thus, the use of a chemical solvent requires a third treatment for removing the water contained in the deacidized gas in order to prevent hydrate formation. This third water removal treatment is often complicated and expensive in the prior art.
One of the objects of the invention is to overcome the problem of removal of almost all of the water initially contained in the natural gas and of reduction, to a commercially acceptable level, of the hydrogen sulfide content, and possibly the carbon dioxide content, of the treated gas while avoiding the drawbacks of the prior art.